The sensitive glass of SiO2 system, Li2O system used for, for example, the pH electrode requires various properties such as small in an alkali error, small in an acid error, a good response, and good chemical durability (in addition, other required properties can be represented by that potential gradient is close to a theoretical value, electric resistance is small, mechanical strength is high, and processing is easy).
Then conventionally various modified metals are blended in order to improve these properties. As an example to improve chemical durability (water resistance), it has been known to include a small amount of La as a trivalent metal in a composition of glass (non patent document 1).
La is filled into a net structure of the glass so as to tighten up on the net and contribute to improvement of water resistance by producing a hydrated gel layer having a constant layer. In addition, La is trivalent and its ionic diameter is relatively large, resulting in small electrostatic force of a univalent anion formed by oxygen square coordination. As a result, it is hardly sensitive to an alkali metal, namely hardly generates an alkali error. An example using another lanthanoid group instead of La has also been known for the same reason.    Non patent document 1: Shinnban pH no riron to sokuteihou (Theory and Measurement Methods for pH (New Edition)) (Maruzen Co. Ltd by H. Yoshimura etc.)    Patent document 1: Japan Patent Laid-open number 2-293343